This invention relates to power amplifiers for controlling the current flow through a load, such as a d-c servo motor. More particularly, the invention relates to power amplifiers of the type comprising a plurality of switching elements connected in bridge fashion with the load and a predetermined power source for coupling the load across alternate output terminals of the power source in accordance with the levels of a plurality of control signals respectively applied to the plurality of switching elements.
Switching power amplifiers of the bridge type normally include diagonally opposed power switching elements which are caused to conduct current through the load (e.g., motor) in a given direction, with one set of diagonally opposed elements causing motor current flow in a forward direction, and the remaining set of diagonally opposed elements allowing motor current flow in the reverse direction. By controlling the conducting intervals of the sets of diagonally opposed switching elements, current flow through the motor can be proportionately controlled. Exemplary bridge type switching power amplifiers employing pulse width modulation (PWM) techniques are disclosed in U.S. Pat. Nos. 3,422,326, 3,471,759 and 3,525,029, and an exemplary so-called "free-running" bridge type switching power amplifier is disclosed in now abandoned U.S. Application Ser. No. 684,522 filed on May 10, 1976 in the names of Frank D. Ruble, et al for CONSTANT CURRENT POWER AMPLIFIER and assigned to the assignee of the present invention.
It is common to provide current feedback signals for stabilizing the overall servo amplifier system of which a switching power amplifier may be part. U.S. Pat. Nos. 3,471,759 and 3,525,029 disclose the use of current feedback for this purpose where the current feedback signals are applied to the servo control portion of the system. It will be appreciated that relying upon the servo control system to correct excessive current flow through the load may cause damage to the load due to the inherent time delay in the servo control operation.
Accordingly, it would be desirable to provide in the switching power amplifier a means for limiting the current flow through the load, thereby overcoming the inherent time delay of the servo control system in correcting for excessive current flow through the load. More specifically, it would be desirable to provide in the power amplifier a means responsive to the output of the current limiter for virtually instantaneously inhibiting further current flow through the load when such current flow falls outside a predetermined range.